An analysis of recently declassified U.S. military documents confirms suspicions that, during the late stages of the Vietnam War, a powerful solar storm caused dozens of sea mines to explode. It’s a stark reminder of the Sun’s potential to disrupt our technological activities in unexpected ways.

As part of Operation Pocket Money, the U.S. Navy planted a series of Destructor sea mines near strategic ports off the coast of North Vietnam. A few weeks later, on August 4, 1972, crew members aboard U.S. Task Force 77 aircraft suddenly observed a batch of explosions south of Hai Phong. In all, some 20 to 30 explosions were documented in just 30 seconds. Another 25 to 30 patches of muddy water were also observed, indicative of further explosions.

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It was a bizarre occurrence, as there was no reason why the mines should have gone off. Almost immediately, U.S. officials began to contemplate extreme solar activity as the cause, as revealed in newly declassified U.S. Navy documents. New research published last month in Space Weather, a publication of the American Geophysical Union, agrees with this 46-year-old assessment, while providing new details about this particularly nasty solar storm, which disrupted more than just naval mines. The study’s authors, led by Delores Knipp from the University of Colorado and Brian Fraser from the National Center for Atmospheric Research, Boulder, say the historical event should serve as a call to action.

The bombs that exploded were magnetic sea mines, a weapon that dates back to the First World War. When a ship passes above, the mine senses a change to the magnetic field density, triggering a detonation. Within days of the August 1972 incident, U.S. military officials began to wonder if solar activity might have been responsible for the unanticipated mine detonations.

As RMIT senior lecturer Brett Carter reports in The Conversation, scientists in the 1970s were already aware of the Sun’s potential to trigger magnetic field changes—they just weren’t sure if it was strong enough to induce the mines into detonating. As part of its investigation, the U.S. military sent officials to the Space Environment Laboratory at the National Oceanographic and Atmospheric Administration (NOAA) near Boulder, Colorado. After consulting with scientists, the investigators concluded with a “high degree of probability” that solar storm activity was responsible for the seemingly spontaneous destruction of the magnetic mines.

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Buried for nearly 50 decades, these now declassified documents were re-analyzed by the Knipp and Fraser team. Indeed, August 1972 experienced an intense period of solar activity—some of the strongest ever recorded.

Between August 2 and 4, sunspot region MR 11976 shot out a series of solar flares, coronal mass ejections, and clouds of charged particles (which were called “plasma drivers” back in the 1970s). The coronal mass ejection that caused the sea mines to explode reached Earth in just 14.6 hours—a record for such an event (it normally takes a full day or two for these electromagnetic pulses to reach our planet’s geomagnetic field and produce magnetic storms). The reason for the speed, the authors say, is that two prior pulses from the Sun on August 2 cleared the path to our planet, resulting in the “ultra-fast” mass ejection on August 4. In addition to the detonation of the mines, the solar storm caused power disruptions and telegraph line outages, as Carter reports.

“Based on the evidence presented, we submit that the 4 August 1972 event was a Carrington-class storm,” the authors write in the study. “The transit time for this event was shorter than the Carrington event.”

By Carrington event, the researchers are referring to a powerful geomagnetic solar storm that occurred in 1859. It remains one of the most powerful solar storms on record. A similar event today would cause severe disruptions, knocking out satellites, power grids, and, as the new study points out, technologies we don’t even know are vulnerable.

In closing, the study authors say the 1972 storm is worthy of further examination and suggest that other researchers to pool their archival information together in an effort to learn more. No doubt, the more reliant we are on technology, the more vulnerable we become to these extreme solar events. Knowing as much as we can about geomagnetic storms could prevent a lot of grief.